Shotguns, particularly invective, which up till the recent update was a really fast firing gun, with little stability. You can't rely on it, nor most other shotguns to yield a reliable amount of damage. I'm putting it up to bad luck that you had to fire three times. You could have misjudged the range too, couple of pellets miss their mark, and he's still not dead. Come back after you've used a more reliable weapon on this thing. I've killed and not killed Warlock Sunsingers up close with shogtuns before. Failing one does not mean that its a universal rule that 2 or three shots don't take them down. Sure, they can have the damage resistance perk, but then they can't self resurrect. And if you encountered him/her and noticed just how mutch damage they did. You can deduce that they were using that particular perk. For a GG, though, three headshots with TDYK will kill 100% of the time.